The invention relates to a method for generating a digital signal from an analog signal generated by means of a frequency converter, and to a frequency converter for implementing the method.
Frequency converters typically generate pulse-width-modulated signals, for example in the form of a (phase) voltage, for driving electric motors, wherein the phase voltage in turn provides a phase current. In the case of frequency converters which feed back the phase current in the context of closed-loop control, the quality of the signal measurement or signal feedback is of high importance.
For this purpose, the signals to be measured are generally picked up by use of suitable sensors, subjected to analog filtering, and then sampled and digitized by use of conventional analog-to-digital converters.
The generally saw-tooth-shaped phase current as one of the relevant signals is generally sampled synchronously in the middle of a respective period of the pulse width modulation. When sampling does not take place precisely in the middle, however, an offset measurement error can be caused.
By averaging over a period of the pulse width modulation, the above-mentioned disruptive effects can be reduced. For the averaging, a plurality of digital measured values is conventionally obtained over a period of the pulse width modulation and then a mean value is calculated over the measured values obtained.
A method in which such averaging is performed is described in DE 10 2007 032 484 A1.
The invention is based on the problem of providing a method for generating a digital signal from an analog signal generated by use of a frequency converter on the basis of pulse width modulation, and a frequency converter for implementing the method, which method and frequency converter are simple and inexpensive to implement and, in particular, enable interference-free and reliable digitization.
The invention solves this problem by providing a method for generating a digital signal from an analog signal, the analog signal generated by a frequency converter on the basis of pulse width modulation, wherein the digital signal corresponds to a mean value of the analog signal over a period of the pulse width modulation. The method comprising the following steps: generating a bit stream in response to the analog signal by a sigma-delta modulator, the bit stream having a predetermined bit repetition duration; and during a time interval having a duration equal to or larger than the period of the pulse width modulation: (i) multiplying a respective bit of the bit stream by a corresponding rating coefficient for generating a respective bit/rating coefficient product, and (ii) summing the respective bit/rating coefficient products, wherein the sum represents the digital signal. A frequency converter, is provided for implementing the method, the frequency converter including: a sigma-delta modulator for generating the bit stream, a memory, which stores the weighting coefficients, a multiplier, which is adapted to multiply a respective bit of the bit stream by a corresponding weighting coefficient, and a summer, which is designed to sum the respective bit/rating coefficient products.
The method serves to generate at least one digital or digitized signal based on an analog signal, for example a phase current and/or a phase voltage, generated by a frequency converter on the basis of pulse width modulation. The signal is used to drive an electric motor, wherein the digital signal, in particular in digital representation, corresponds to a mean value of the analog signal over a period of the pulse width modulation. The digital signal is used in particular for the closed-loop control of the analog signal, for example in a servo-converter. According to the method, a continuous bit stream having a predetermined, in particular constant, bit repetition duration is generated based on the analog signal, in particular by a sigma-delta modulator. During a respective time interval which has a duration which is equal to or longer than the period duration of the pulse width modulation, the following steps are performed: multiplying a respective bit of the bit stream by a corresponding rating coefficient to generate a respective bit/rating coefficient product, and summing all of the bit/rating coefficient products over the time interval, wherein the sum over the time interval represents the digital signal. The above-mentioned steps are repeated continuously, for example for each period or every n-th period of the pulse width modulation, where n≧2.
A digital representation of a signal or a digital signal is understood to mean a sequence of time-discretely and amplitude-discretely digitized sampled values, which firstly have a discrete and quantized set of values and, in addition, are defined in terms of the temporal sequence only at specific periodic times or demonstrate a change in the sampled value. The signal with a digital representation is formed from an associated analog signal, which describes the temporally continuous profile of a physical variable, by quantization and sampling, which is performed at different points-in-time. By corresponding encoding, a digital signal can be converted into a binary representation. Moreover, reference is also made to the extensive literature in the art.
In one development, the rating coefficients are generated by superimposing weighting coefficients of a weighting function, wherein a midpoint of a respective weighting function lies within a period of the pulse width modulation, a respective weighting function corresponds to a digital filter function, adjacent weighting functions are shifted in time with respect to one another by one or a multiple of the bit repetition duration, and a respective weighting function has a predetermined number of weighting coefficients. Preferably, the time interval has a duration which is longer than a period of the pulse width modulation by a number of bit repetition durations, wherein the number of bit repetition durations corresponds to the number of weighting coefficients.
In one development, the time interval is arranged centrally with respect to a period of the pulse width modulation.
In one development, the digital filter function is a sinc filter function.
The frequency converter, in particular in the form of a servo-converter, is designed to implement the abovementioned method and comprises: a sigma-delta modulator for generating the bit stream, a memory which stores the weighting coefficients, a multiplier which is designed to multiply a respective bit of the bit stream by a corresponding weighting coefficient, and an adder, which is adapted to sum the respective bit/rating coefficient products.
The invention will be described below with reference to the drawings which illustrate preferred embodiments of the invention schematically as follows.